The present invention relates generally to charge pumps and more specifically to an external tri-state charge pump for extending the steering line output range of a charge pump that is internal to a synthesizer integrated circuit.
Synthesizer integrated circuits (xe2x80x9cICsxe2x80x9d) are used in a number of different devices. These include two-way radios, wireless personal communication devices, wireless automotive devices, and home and entertainment devices requiring a stable radio frequency to transmit or receive audio, video, and data signals. Some prior art synthesizer ICs have phase detector (or dual) outputs that are used to control existing external charge pump designs for use in applications that require a range of control voltage values. As die shrinks and new processes are developed to reduce IC manufacturing costs, synthesizer IC""s with phase detector outputs are being phased out to reduce pin count and package size. The synthesizer ICs with phase detector outputs are being replaced by synthesizer ICs with internal charge pumps. However, current synthesizer ICs with an internal charge pump have a reduced maximum voltage, which may be insufficient for many applications such as high tier base stations, mobile, and portable two-way radios.
FIG. 1 illustrates a simple block diagram of a typical phase locked loop (xe2x80x9cPLLxe2x80x9d) circuit 100, such as may be used in wide range of transmitting and receiving devices described above. PLL circuit 100 comprises a synthesizer IC 110 with an internal charge pump (not shown). Synthesizer IC 110 has a prescaler input 112 and a single pin output 114 that is a steering line output from the internal charge pump. The output signal at output 114 is typically a minimum voltage value, a maximum voltage value or a tri-state output. PLL circuit 100 further comprises an external charge pump circuit 120, having an input 122 coupled to synthesizer output 114, and having an output 124. External charge pump circuit 120 may be, for instance, a prior art external charge pump design or a charge pump circuit in accordance with the present invention. PLL circuit 100 also includes a loop filter 130 coupled to the charge pump output 124. Depending upon the application and circuit arrangement, loop filter 130 may include circuit components such as one or more resistors, and one or more capacitors depending on the order of the filter. In addition, although loop filter 130 is illustrated as being external to charge pump 120, those of ordinary skill in the art will realize that the loop filter may be integrated into the charge pump. Finally, PLL circuit 100 includes a voltage-controlled oscillator (xe2x80x9cVCOxe2x80x9d) 140 coupled between the loop filter 130 and synthesizer input 112 via a VCO feedback circuit 160, and also includes a PLL output 150.
FIG. 2 is a circuit diagram illustrating a PLL circuit 200 having a prior art charge pump circuit. PLL circuit 200 comprises a typical synthesizer IC 210 with an input 212 and an output 214. Synthesizer IC 210 includes an internal charge pump (not shown) that generates a maximum steering line voltage at output 214 that is insufficient for many applications, as noted above. PLL circuit 200 further comprises a prior art external charge pump for extending the steering line range at output 214. This external charge pump includes an operational amplifier (xe2x80x9cop-ampxe2x80x9d) 220, resistors 222 and 224 and a loop filter 230, which includes a resistor 238 and capacitors 232, 234 and 236. The charge pump functions in a voltage mode using op-amp 220 as a voltage-to-voltage converter to generate the increased steering line range. Finally, PLL circuit 200 includes a VCO 240 coupled between op-amp 220 and synthesizer input 212 via a VCO feedback circuit 260, and also includes a PLL output 250.
A shortcoming of the above prior art charge pump is that it requires an op-amp, which is inherently noisy and raises the cost of the device. Another shortcoming of the prior art charge pump design is that although this design is acceptable at room temperatures, it fails over environmental extremes, e.g., humidity, temperature, etc. Still another shortcoming of the prior art charge pump design is that it operates in only two states, and does not address the tri-state output of the synthesizer IC.
Thus, there exists a need for an external charge pump that extends the upper limit of a synthesizer IC""s internal charge pump steering line range, that operates over environmental extremes, and that operates to address the tri-state output of the synthesizer IC.